Depending upon the circuit board application, purchasers of electronic printed circuit boards (PCBs) may require no more than routine reliability or, in sharp contrast, may require a very high degree of reliability. A toy having a PCB would be in the former category while military aircraft, certain automotive applications and the like would clearly be in the latter.
One way to help assure a high level of reliability (or, conversely, to cull out PCBs which may exhibit "infant mortality" or premature failure) is to "stress screen" them. Such screening is by placing the PCBs in a chamber, the interior air temperature of which is repetitively cycled between about 120.degree. C. and about -60.degree. C. over a relatively-short period of time, i.e., well less than one hour or so.
In one exemplary test chamber arrangement, the chamber has a removable wall mounted on and forming a component of a wheeled cart. That side of the wall at the chamber interior has a fixture for mounting PCBs thereon during test. On the opposite side of the wall, the cart has a rack on which instruments or the like are mounted. Feedthrough devices extend through separate respective openings in the removable wall and connect to PCBs in the fixture and to the instruments on the rack. This arrangement is used in a known test method including connecting the PCBs to instruments during stress screening--no switching is involved. In the parlance of the industry, this is known as Class A testing.
Another known PCB testing method includes switching the PCBs between a load board and a signal bus while such PCBs are undergoing thermal stress screening. Individual relays are used to switch each electrical lead. Since each relay can handle but a single circuit, the number of relays required is, in general, equal to the number of wires or leads extending from the PCBs under test. Thus, installations with more than 100 relays are common. Relays can be troublesome, both from the standpoint of undesirably-high contact resistance and with respect to reliability.
Heretofore, those conducting "switched" testing as well as non-switched or Class A testing purchased (and stored and maintained) two carts, one configured for PCB switching and the other configured for Class A testing. A leading manufacturer of such carts and related equipment is RPI, Inc. of Racine, Wisconsin.